1. Field of the Invention
The present invention relates to an anode for electrolysis which possesses high oxygen overvoltage characteristics and excellent anticorrosive properties.
2. Description of the Prior Art
Heretofore, alkali metal halide electrolysis such as the electolysis of sodium chloride has been chiefly conducted by the mercury process. However, the drainage from the mercury process is a source of pollution. Workable alternatives to the mercury process are the diaphragm process or the ion-exchange membrane process. In the diaphragm process, the current density is less than that of the mercury process during the operation. However, the electrodes employed in the process have a low oxygen over-voltage. Thus, when these electrodes are used in the diaphragm process or the ion-exchange membrance process, about 1 to 5 % oxygen is included in the chlorine produced. Because of the presence of oxygen, the anolytic gas cannot be directly fed to a petrochemical plant. In order to use the anolytic gas it is necessary to remove oxygen. The oxygen removal process is complicated, which increases the cost of the gas.
The platinum group elements are believed to be catalysts for the electrode. In the electrolysis of an aqueous solution of an alkali metal salt, the chlorine overvoltage, oxygen overvoltage and anticorrosive properties are of the following order.
Chlorine overvoltage: EQU Pd.apprxeq.Ru&lt;Ir&lt;&lt;Rh&lt;&lt;&lt;Pt
Oxygen overvoltage: EQU Ru&lt;Ir&lt;Rh&lt;Pd&lt;Pt
Anticorrosive property: EQU Ru&lt;Pd&lt;Pt&lt;Ir&lt;Rh
In general, when a platinum group element is used, there is no catalyst for the electrode which has balanced low chlorine overvoltage, high oxygen overvoltage and high anticorrosive properties. If one property is satisfactory, then another property is not satisfactory. In order to improve the conventional electrodes, various combinations of Pd, Ru or Ir which have low chlorine overvoltage and Pt, Ir or Rh which have substantial anticorrosive properties have been studied. However, these combinations have not possessed either satisfactory anticorrosive properties or satisfactory oxygen overvoltage.
Among the platinum group metals, palladium seems to be optimum from the viewpoint of the slow rate of generation of oxygen and the level of catalytic activity of the electrode for the chlorine electrode reaction. However, when metallic palladium is coated on the electrode, it possesses poor anticorrosive characteristics, and consequently, it dissolves during electrolysis. Thus, this type of electrode is not practically useful. If the electrode is coated with palladium oxide (PbO), the adhesiveness of the membrane is inferior because of differences between the crystalline form of palladium oxide and the crystalline form of titanium oxide which result upon oxidation of the substrate.
In the present invention various combinations of platinum oxide and tin oxide have been studied. As a result, the electrolysis electrode which is prepared by coating a membrane of iridium oxide and tin oxide or a membrane or ruthenium oxide and tin oxide on a conductive substrate could not be practically used because when an oxygen overvoltage greater than 0.6 Volt is obtained at the current density of 20 mA/cm.sup.2 to decrease the generation of oxygen gas, the chlorine overvoltage is higher than 0.1 Volt. While the combination of platinum oxide and tin oxide exhibits excellent initial characteristics it could not be practically used because of substantial variations in ageing characteristics. However, it has now been found that the combination of 5 to 40 mole % of palladium oxide and 95 to 60 mole % of tin oxide achieves a satisfactory oxygen overvoltage, chlorine overvoltage and anticorrosive property. Thus, further improvement in an electrode having balanced characteristics has been attained.